Untitled Flashcards Set

Parental Care and Fecundity

Fecundity is the potential reproductive capacity of an individual within a population. In other words, fecundity describes how many offspring could ideally be produced if an individual has as many offspring as possible, repeating the reproductive cycle as soon as possible after the birth of the offspring. Since individuals have a finite energy budget, there is typically a trade-off between investment in fecundity versus parental care. For example, species that produce many offspring (e.g., many marine invertebrates) usually provide little if any care for the offspring since most of their energy budget is used to produce many tiny offspring. Animals with this strategy are often self-sufficient at a very early age, but their small size makes them extremely vulnerable to predation. Conversely, animal species that have few offspring during a reproductive event usually give extensive parental care, devoting much of their energy budget to these activities. The offspring of these species are relatively helpless at birth and need to develop before they achieve self-sufficiency, but their mortality rates early in life are generally low because the parents protect them from predation.

Environmental Limits to Population Growth (45.3)

Although life histories describe the way many characteristics of a population, a variety of quantitative methods are used by population ecologists  to model population dynamics and predict future changes in population size.

Exponential Growth

Charles Darwin, in his theory of natural selection, was greatly influenced by the English clergyman Thomas Malthus. Malthus published a book in 1798 stating that populations with unlimited natural resources grow very rapidly, which represents an exponential growth, and then population growth decreases as resources become depleted, indicating a logistic growth.

The best example of exponential growth is seen in bacteria. Bacteria reproduce by prokaryotic fission. This division takes about an hour for many bacterial species. If 1000 bacteria are placed in a large flask with an unlimited supply of nutrients (so the nutrients will not become depleted), after an hour, there is one round of division and each organism divides, resulting in 2000 organisms—an increase of 1000. In another hour, each of the 2000 organisms will double, producing 4000, an increase of 2000 organisms. After the third hour, there should be 8000 bacteria in the flask, an increase of 4000 organisms. The important concept of exponential growth is that the number of organisms added in each reproductive generation increases over time. Under this growth model, when the population size (N) is plotted over time, a J-shaped growth curve is produced (Figure 45.9). 

Logistic Growth

Exponential growth is possible only when infinite natural resources are available; this is not the case in most real-world situations. Charles Darwin recognized this fact in his description of the “struggle for existence,” which states that individuals in a population will compete for limited resources. To model the reality of limited resources, population ecologists developed the logistic growth model.

Carrying Capacity and the Logistic Model

In the real world, with its limited resources, exponential growth cannot continue indefinitely. Exponential growth may occur in environments where there are few individuals and plentiful resources, but when the number of individuals gets large enough, resources will be depleted, slowing the growth rate. Eventually, the growth rate will plateau or level off (Figure 45.9). This population size, which represents the maximum population size that a particular environment can support, is called the carrying capacity, or K.

The formula we use to calculate logistic growth adds the carrying capacity as a moderating force in the growth rate. That is, the growth of the population slows as the population size approaches the carrying capacity. A graph of population growth under the logistic model yields an S-shaped curve (Figure 45.9), and it is a more realistic model of long-term population growth than exponential growth. There are three different sections to an S-shaped curve. Initially, growth is exponential because there are few individuals and ample resources available. Then, as resources begin to become limited, the growth rate decreases. Finally, growth levels off at the carrying capacity of the environment, with little change in population size over time.

Figure 45.9 When resources are unlimited, populations exhibit exponential growth, resulting in a J-shaped curve. When resources are limited, populations exhibit logistic growth. In logistic growth, population expansion decreases as resources become scarce, and it levels off when the carrying capacity of the environment is reached, resulting in an S-shaped curve.

Life Histories of K-selected and r-selected Species

These models of population growth are related to the regulation of population size, competition, and the life histories of species. Population biologists have grouped species into the two large categories — K-selected and r-selected — although the categories are really two ends of a continuum.

K-selected species are species selected by stable, predictable environments. Populations of K-selected species tend to exist close to their carrying capacity (hence the term K-selected) where intraspecific competition is high. These species have few, large offspring, a long gestation period, and often give long-term care to their offspring. Examples of K-selected species are primates (including humans), elephants, and plants such as oak trees (Figure 45.13a). In contrast, r-selected species have a large number of small offspring (hence their r designation (Table 45.2)). This strategy is often employed in unpredictable or changing environments. Animals that are r-selected do not give long-term parental care and the offspring are relatively mature and self-sufficient at birth. Examples of r-selected species are marine invertebrates, such as jellyfish, and plants, such as the dandelion (Figure 45.13b).

Figure 45.13 (a) Elephants are considered K-selected species as they live long, mature late, and provide long-term parental care to few offspring. Oak trees produce many offspring that do not receive parental care, but are considered K-selected species based on longevity and late maturation. (b) Dandelions and jellyfish are both considered r-selected species as they mature early, have short lifespans, and produce many offspring that receive no parental care.

Human Population Growth (45.5)

Population dynamics can be applied to human population, which is currently experiencing exponential growth (Figure 45.14). Long-term exponential growth carries the potential risks of famine, disease, and large-scale death, and there is some worry about the ability of the earth’s environment to sustain this population.

Figure 45.14 Human population growth since 1000 AD is exponential (dark blue line). Notice that while the population in Asia (yellow line), which has many economically underdeveloped countries, is increasing exponentially, the population in Europe (light blue line), where most of the countries are economically developed, is growing much more slowly.

A consequence of exponential human population growth is a reduction in time that it takes to add a particular number of humans to the Earth. For example, it took approximately 123 years to change from 1 to 2 billion people (1927), 33 years to change from 2 to 3 billion (1960), and 14 years to change from 3 to 4 billion (1974). Since that milestone the growth rate has leveled off at about 1 billion per 11-12 years, and the population size reached 8 billion in 2022.

Age Structure, Population Growth, and Economic Development

The age structure of a population is an important factor in population dynamics. Age structure is the proportion of a population at different age ranges. Age structure allows better prediction of population growth, plus the ability to associate this growth with the level of economic development in the region. Countries with rapid growth have a pyramidal shape in their age structure diagrams, showing a preponderance of younger individuals, many of whom are of reproductive age or will be soon (Figure 45.16). This pattern is most often observed in underdeveloped countries. Age structures of areas with slow growth, including the United States, still have a pyramidal structure, but with many fewer young and reproductive-aged individuals and a greater proportion of older individuals. Other developed countries, such as Italy, have zero population growth. The age structure of these populations is more conical, with an even greater percentage of middle-aged and older individuals. The actual growth rates in different countries are shown in Figure 45.17, with the highest rates tending to be in the less economically developed countries of Africa and Asia.

Figure 45.16 Typical age structure diagrams are shown. The rapid growth diagram narrows to a point, indicating that the number of individuals decreases rapidly with age. In the slow growth model, the number of individuals decreases steadily with age. Stable population diagrams are rounded on the top, showing that the number of individuals per age group decreases gradually, and then increases for the older part of the population.